An Electrical Wiring Diagnostic Tip
Jun 27, 2013
Summertime is trailer time--boat trailers, camping trailers, livestock trailers, etc., etc. And anybody who pulls a trailer will eventually spend time trying to diagnose electrial problems in the trailer's wiring system. Here's a tip:
The old school way to test for voltage in a wire was to scrape the rust or paint from a nearby frame component and ground the voltmeter or test light to the frame. That test worked okay when lights were wired with one power wire to the bulb, and ground was achieved via the light's mounting bracket or a short jumper wire from the light to the frame.
But modern trailer wiring harnesses are often wired with a ground circuit that goes all the way back to the towing vehicle's negative battery post. If the mechanic tests for voltage at the light socket by grounding his voltmeter/test light to the nearby frame, it bypasses the ground circuit back to the battery. Grounding the test meter to the nearby frame only tests half of that sort of circuit.
For example, let's say that ground wire is broken. If the mechanic tests the positive wire to the light and grounds his voltmeter or test light to the frame near the light socket, it shows 12 volts. But because the ground wire back to the battery is broken, the circuit is open and the light doesn't illuminate, even though there is technically "12 volts to the light socket"--when a temporary ground is provided.
It takes a little detective work, some deductive reasoning, to figure out if an electrical problem is in the positive wires or the negative wires of a circuit that shows zero volts. But that's better than grounding to frame at the light socket, showing 12 volts, and then wondering, "If I've got 12 volts at the light socket, why doesn't the light work?"